On Mon, 17 Nov 2008 08:23:18 -0800, Nate Certified Heating and Air
Professional wrote:

On Mon, 17 Nov 2008 10:46:47 -0500, wrote:

I have 3 new heat pump systems in my new well insulated home. Two of the
systems were working fine. The other system was had been set to 68 for
several days. This morning it was 36 outside, the actual temperature
registered on the theromastat was 68 and the aux heat was on.

My HVAC company is telling me it is normal for the aux heat to be on when
it is below 40 outside. I am questioning if that is correct or not. I
thought heat pumps worked well until the outside temperature was about 20
degrees.

Is my HVAC company correct? If not, at what outside temp should the aux
heat be necessary in a well designed properly working system? Can you
provide any reasonably authoritive info on the web that I can show to them?

Many thanks for any help.


There are a couple of instances when heat strips come on;

1. When the unit goes into defrost mode.

2. When the user adjusts the thermostat to be 3 to 5 degrees (depends
on the stat) greater than the indoor temperature that is indicated on
the thermostat.

3. When there is a outdoor thermostat (connected to the outdoor unit)
that senses when the outdoor temp goes below a preset temperature.

4. When the thermostat is miswired.

5. When the defrost control board malfunctions.

I suspect that your installers are right and that the unit in question
has an outdoor thermostat and is doing what it is supposed to do as no
heat pump will heat efficiently at 20 degrees outside air temp, most
start loosing efficiency at around 35 degrees. I would want to know
why the other two are not doing the same thing.

The units do not have an external thermostat. In case it helps, the units
are American Standard. The thermostats are "ACONT800 Series Touch Screen
Programmable Comfort Control" units.